The most common and most likely answer to this crossword clue is the 6 letter word ORACHE
I've seen this crossword clue in Independent
O
R
A
C
H
E
Buddy explains!
The straight part
We know that "This could replace spinach" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this could replace spinach = orache.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ORACHE mean?
ORACHE noun- The saltbush: any of several plants, of the genus Atriplex, especially Atriplex hortensis or Atriplex patula, found in dry habitats, that have edible leaves resembling spinach.